As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material, and which is covered on both sides with layers of thermoplastic material, e.g. polyethylene film. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of oxygen-barrier material, e.g. aluminium foil, which is superimposed on a layer of thermoplastic material, and is in turn covered with another layer of thermoplastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections to form pillow packs, which are then folded mechanically to form respective finished, e.g. substantially parallelepiped-shaped, packages.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the packages are filled with the food product and sealed. One example of this type of package is the so-called “gable-top” package known by the trade name Tetra Rex (registered trademark).
Once formed, the above packages may undergo further processing, such as the application of a reclosable opening devices to protect the food product inside the package from contact with external agents, and to enable the product to be poured out.
At present, the most commonly marketed opening devices comprise an annular frame portion defining a pour opening and fitted about a removable or pierceable portion of a top wall of the package; and a cap hinged or screwed to the frame portion, and which is removable to open the package. Alternatively, other types of opening, e.g. slide-open, devices are also known to be used.
The removable portion of the package may be defined by a sealing sheet glued or heat-sealed to the outside of the package to close a through hole in the package. One example of this solution is described and illustrated in Patent Application EP-A-9433549. Alternatively, the removable portion of the package may be defined by a so-called “prelaminated” hole, i.e. a hole formed in the base layer of the packaging material before covering the base layer with other layers defining the packaging material, e.g. the layers of thermoplastic material and/or the layer of barrier material, which close the hole hermetically. One example of this solution is described and illustrated in Patent Application EP-A-331798.
In both cases, before being applied to the respective packages, the opening devices are fed successively through a gluing unit, in which they are coated with adhesive, usually hot-melt glue.
Gluing units are known which substantially comprise a conveyor for feeding the opening devices along a given path; and an adhesive dispenser, which interacts with each opening device to apply adhesive to one or more specific areas of a fastening portion of the opening device.
More specifically, the adhesive is applied by stopping the conveyor and operating the dispenser along a given deposition path, so as to apply adhesive to a first substantially oval-shaped area, and a second spot area, lying within the first area, of the opening device.
In particular, to ensure the adhesive glues the opening device firmly to, and seals, the area of the package to which the opening device is applied, at least part of the deposition path must be covered more than once.
Though reliable and efficient, the gluing units described still leave room for further improvement, particularly as regards stepping up output.
In particular, the output of known units is limited by the speed at which the dispenser travels along the deposition path having to allow the adhesive to interact with each opening device long enough to reduce the formation of adhesive trickle, between the dispenser and the opening device, which would impair efficiency of the opening device and call for frequent cleaning of the dispenser, thus reducing output.